Partially allow overriding of `String#[]` methods

This is for the purpose of supporting `Regexp`.

- configuration and build

  ```console
  % cat regexp_config.rb
  MRuby::Lockfile.disable
  MRuby::Build.new do
    toolchain "clang"
    enable_debug
    gem core: "mruby-bin-mruby"
    gem core: "mruby-print"
    gem mgem: "mruby-onig-regexp"
    #gem mgem: "mruby-regexp-pcre"
  end

  % rake MRUBY_CONFIG=regexp_config.rb
  ```

- mruby HEAD (bec074e)

  ```console
  % build/host/bin/mruby -e 'p "abcdefg"[/.(?=...$)/]'
  -e:1: can't convert OnigRegexp into Integer (TypeError)
  ```

- with this patch

  ```console
  % build/host/bin/mruby -e 'p "abcdefg"[/.(?=...$)/]'
  "d"
  ```

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